1. Field of the Invention
The present invention relates to an accelerometer and, more particularly, to a thermally stabilized accelerometer.
2. Description of the Prior Art
FIG. 1 is a cross sectional view showing an example of a prior art accelerometer, and FIG. 2 is a plan view thereof partially sectioned.
In the prior art device, a rod-like pendulum 1 is provided to detect acceleration. The pendulum 1 is supported on a hinge 3 having a central flexible or a thin joint portion so as to swing about the joint portion 2 only in a direction indicated by the arrows X--X parallel to the surface of the drawing sheet. One end of the hinge 3 is firmly fixed to the base 5 of pot-shaped frame 4 by a pressure plate 6, screw 6A, or the like. The pendulum 1 has attached to it two cylindrical torquer coils 7 and 7' whose planes are perpendicular to the swing axis of the thin joint portion 2, i.e. axis 0--0. A disk-like permanent magnet 8 and a column-shaped pole piece 9 are inserted in and attached to the frame 4 by adhesive material, for example, and spaced from the wall of the frame 4 to allow the torquer coil 7 to be inserted therebetween without contact. The first frame 4 is made of electromagnetic soft-iron so that it forms an electro magnetic circuit with the pole piece 9 and the permanent magnet 8. Thus constructed, the frame 4 serves as a yoke (return path) for the magnet flux and the entire unit constitutes a first torquer defined by the numeral 30. This first torquer is equipped with apparatus for detecting the displacement of the pendulum 1 comprising two light receiving elements 10, 10' and a light emitting element 11 mounted on the frame 4.
The illustrated accelerometer has a second base or a frame 12 made of electromagnetic soft-iron, as is the first frame 4. A permanent magnet 13 and a column-shaped pole piece 14 are attached within the pot of the frame 12 to allow insertion the torquer coil 7' without contact therewith. The second frame 12 also serves as a yoke, and the assembly constitutes a second torquer magnet generally defined by the numeral 31. An engaging portion 16 is formed on the second frame 12 for insertion inserted into an opposing engaging portion 15 formed on the first frame 4 so that the frames can be made into a unitary body.
The first frame 4 and the second frame 12 are provided with adjustable stoppers (not shown) which limit the range of movement of the pendulum 1. When the stoppers are respectively rotated and moved by a screw, the range can be freely adjustable.
The above-mentioned components function as an accelerometer. They may be fixed in a housing formed by the frame 4 and a casing 23 by a plurality of screws, and kept in airtight condition by a cover 25 over the opening 24.
External terminals 22 extend through the housing 23 for connection to lead lines (not shown) or the like by which an electrical signal from the light receiving elements 10, 10' can be supplied. The light receiving elements 10, 10' constitute the displacement detector and a feed-back current source to the torquer coils 7, 7' After wiring the electronic system within the terminal case 23, the opening 24 is closed by the lid 25, and the interior of the case 23 is placed under vacuum or filled with inert gas (for example, helium) and kept in airtight condition to avoid deterioration of the parts and maintain the assembly stable for a long period of time.
When acceleration is applied to the above described accelerometer in the X--X direction, the pendulum 1 will be slightly displaced, with the thin joint portion 2 of the hinge 3 as the center of rotation. Thus, displacement of the pendulum 1 produces a change in the incident light received by light receiving elements 10 and 10', causing an electrical output signal corresponding to the displacement of the pendulum 1 to be created. This electrical signal is fed to the torquer coils 7, 7', changing the flux pattern and causing the pendulum 1 to be restored to its initial such that the output electrical signals from the light receiving elements 10 and 10' are made zero. This electrical current, being a restoring current, is in proportion to the acceleration so that it also functions as an input current by which acceleration can be measured.
By the way, in FIG. 2, numeral 19-3 indicates an attaching aperture for attaching the above-mentioned acceleration to the body whose acceleration is to be measured.
This prior conventional accelerometer cannot, however, provide high accuracy measurement because the magnetic motive force of the permanent magnet 8 varies in response to the change of external temperature. As a result, the ratio of input acceleration to output signal, in other words the scale factor, is influenced by temperature change.